Magnetoelastic resonators have been used in sensing applications due to certain properties of magnetoelastic materials. In the presence of a magnetic field, mechanical strain is induced in a magnetoelastic material. The induced strain results in the production of additional magnetic flux, which can be detected wirelessly. In addition, magnetoelastic resonators typically operate at a specific resonant frequency, which limits interference from spurious sources. Magnetoelastic resonators are also passive devices, meaning that they require no power sources or circuits to function. These characteristics—wireless operation, signal isolation, and passivity—make magnetoelastic resonators attractive in applications for remotely detecting, locating, or mapping items. Potential applications include, for example, tagging of inventory, wirelessly detecting blockage or leakage in piping systems or sophisticated machines, and sensing parameters in medical implants.
In one commercially successful application, magnetoelastic tags are used in electronic article surveillance (EAS) systems. Magnetoelastic tags have gained some acceptance over RF and magneto-harmonic tags in such systems, which are employed as theft-deterrent systems in libraries, supermarkets, retail stores, etc., due in part to an attractive price/performance ratio. In such systems, a rectangular strip of magnetoelastic material and a bias magnet are sandwiched between other material layers. An interrogation coil and a receiving coil are positioned at the store exit, for example, with the interrogation coil providing a magnetic field at the resonant frequency of the tag. When the tag passes between the interrogation and receiving coils, the tag resonates and induces an additional signal in the receiving coil. When the system detects the additional signal, an alarm may be activated. The bias magnet is simply demagnetized when a product is paid for or otherwise permissively taken, effectively deactivating the resonator.
Though such passive, wireless detection schemes may be desirable in many other applications, the relatively large size of commercially available magnetoelastic tags makes them impractical for many applications. For instance, a typical commercial magnetoelastic tag operating at 58 kHz is about 38 mm long, 12.7 mm or 6 mm wide and 27 μm thick. Smaller tags operating at 120 kHz, with adequate signal strength for commercial use, still have a length of about 20 mm and width of 6 mm. These magnetoelastic tags are usually strips or ribbons and the length-to-width ratio is normally larger than 3:1. Despite great improvements in signal strength and detection range provided by advancements in magnetoelastic material properties and optimized detection approaches, successful miniaturization of magnetoelastic tags has not been realized. Because response signal strength is proportional to the effective volume of magnetoelastic material, smaller resonators result in smaller signals that are more difficult to detect. In addition, the dimensional tolerances and variation present in conventional magnetoelastic material manufacturing have a greater effect on smaller resonators.